In the present coating, the ratio between the thickness of the metal-free DLC layer and the metal layer is greater than 4 and/or of the thickness ratio of the overall layer by the thickness of the outer layer (a-C:H) is greater than 1, preferably between 1 and 1.4. The adhesive layer should preferably be chromium. The metal DLC layer designated as a-C:H:Me and, in this case, the metal is preferably tungsten termed as a-C:H:W, this layer has a multilayer structure comprising a-C:H layers and a-C:H:W layers.
The tungsten promotes excellent adhesion between the DLC layers and functions as an intermediate layer gradually changing the internal stress minimizing an abrupt change of the internal stress of the base material (internal low stress) to the DLC (high internal stress), additionally having a high chemical inertness and low wear. The outermost or top layer presents an amorphous structure containing carbon and hydrogen and can be called a-C:H.
Due to the aforementioned constitutions, especially beneficial properties with regard to friction and wear were achieved. These typical properties have been influenced by a thicker top layer. When compared to the average thickness layers, it has lower friction and greater durability. Particularly good friction values were observed when the outermost layer (a-C:H) had a thickness greater than the DLC layer containing metal, so that the thickness ratio of both is close to 5, preferably greater than 4, and the thickness ratio of the total layer with the thickness of the outer layer (a-C:H) is greater than 1, preferably between 1 and 1.4.
This configuration allowed solving two problems, the friction reduction and durability of the DLC layers. As an explanation for this behavior, it was discovered that the metal-free DLC layer and the general coating generate high stress in the system as a whole, which can be reduced with the correct combination of the thickness of the metal layer and the outermost layer, and of the configuration of intermediate layers of chromium and tungsten carbide (WC1-x), so that a coating having an improved combination of wear resistance and low friction is obtained.
Thus, one obtains a piston ring with proper durability and low friction. It is known that metal-free DLC has a higher wear resistance and low friction. However it is not possible to produce a thick coating of metal-free DLC because of high internal stresses of this coating and, with the configuration proposed in this document, it was possible to build up a thick layer of metal-free DLC without it being weakened by high stresses, with a ratio between the layer thickness of metal-free DLC layer and the metal layer of >4 and/or of the thickness ratio of the total layer with the thickness of the outer layer (a-C:H) is greater than 1, preferably between 1 and 1.4, which yields the piston ring a longer life and lower friction than that secured by an outermost DLC layer, which guarantees a low wear resistance with a low lifetime exposing the metal DLC layer at the start of operation of the engine, such that this layer has friction higher than that of the metal-free DLC layer. The sliding element thus configured has advantages such as increased stability and durability, giving the internal combustion engine equipped with it a large commercial service life and low friction.
In contrast, if coating is formed in a layer thickness ratio between the metal-free DLC layer and the metal DLC layer of <4 and/or the ratio with the total thickness with the thickness of the outer layer (a-C:H) is <1, the residual stresses will not be compensated. This leads to premature wear of the DLC layer as a whole, despite the large thickness of the outermost layer or a peeling of the DLC layer due to high loads (contact pressure) during operation.
Recent and extensive studies conducted by the applicant have confirmed that, regardless of thickness substantially between 4 μm and 15 μm, the present coating results in interesting and advantageous properties of wear resistance and durability.
For an internal combustion engine to function reliably and in accordance with the parameters devised by its designers, it is necessary that its internal components have high durability even under the most critical operating conditions.
In order to ensure the durability of an engine and an efficient yield, components such as piston rings and bearings, among others, must provide resistance to wear originated from constant sliding, high temperatures and chemical and abrasive attack from products of combustion occurring within the cylinders, and thereby receive coatings to better withstand the endless cycles of engine operation. In addition, this coating should have low friction since friction losses in a combustion engine represent up to 15% of energy and consequent higher fuel consumption and higher emissions of polluting gases.
The improvement of coatings applied to these components is constant, having as premises the base material that the component is made of, the operating parameters of the engine, the manufacturing costs, etc.
Specifically with respect to piston rings, some coatings were developed aiming to give these components high durability and low friction.
A first coating is disclosed in document DE 10 2009 046 281, which relates to a piston ring made of steel or cast iron provided with a coating consisting of a metal-free DLC layer or comprising an inner metal layer and a metal-free top layer, a chromium nitride (CrN) layer deposited by the PVD process (“Physical Vapour Deposition”) and a ceramic intermediate layer of Me(CxNy).
The nitrided layer is applied to the ring base and over it, subsequently, the aforementioned intermediate layer is applied. Finally, the DLC coating is applied over the intermediate layer.
As an option (not required) an adhesive layer is provided which makes the connection between the chromium nitride layer applied by PVD and the base of the piston ring or the like.
A vulnerability of this coating lies in the fact that the Me (CxNy) link layer is fragile and brittle and can lead to premature detachment of the coating, resulting in a lower lifetime of the internal combustion engine, which is undesirable in terms of market.
Moreover, contrary to the proposed coating, this prior art coating has no transition layer comprising tungsten carbide WC1-x, making it, therefore, quite different.
Document DE 10 2008 042 747 discloses a sliding element such as a piston ring whose coating comprises an adhesive layer consisting of metal chromium applied onto the substrate, a nitride layer (CrN) applied by the PVD process (Physical Vapour Deposition), an internal layer provided with carbon that has greater hardness and/or contains a small percentage of hydrogen relative to a carbon layer provided outside preferably of the type a-C:H.
The disadvantage of this second coating lies in the fact that hydrogen favors the formation of the sp3 electronic structure (same structure of diamond) and with its reduction there will be an increase of the sp2 structure (same structure of graphite). With this, the wear resistance of the coating is impaired.
Moreover, as the crucial difference related to the coating developed herein by the applicant, this prior art coating has, as the great advantage, the second layer, counted from the outside inwards, which has the form of amorphous carbon instead of the novel multilayer structure (a-C:H: W) and (a-C:H). Due to this formation, the indices of performance achieved by this coating are lower, especially in the resistance to crack propagation.
Document DE 10 2009 028 504, in turn, discloses a piston ring manufactured in steel or cast iron initially coated with an adhesive layer to which is applied an intermediate layer containing carbon and a metal (especially Tungsten) and a DLC layer without the occurrence of a metallic component. The average thickness of this coating is 5 μm to 40 μm, the ratio between the thicknesses of the outer layer and the intermediate layer is 0.7 to 1.5 and the ratio between the thicknesses of the outer layer and the total thickness of the coating is about 0.4 to 0.6.
As the big difference relative to the coating developed herein, this prior art does not show any middle layer configured on multilayer structure (W-C:H) and (a-C:H). In fact, this layer is absent from this coating and, thus, the indices of performance achieved by this coating are low, especially in the resistance to crack propagation. Additionally, the thickness of the metal-free DLC coating, which is the layer that provides lower friction, has a thickness lower than the total coating.
Finally, document US 2007/0078067 discloses a coating applied to at least one sliding member characterized in that it comprises a film of amorphous carbon on the surface of the sliding member and has a D band (associated with the disorder of the sp2 carbon) to G band (monocrystalline graphite) integrated intensity ratio in the Raman spectrum between 1.5 and 2.0.
The coatings discussed above have properties which make them unsatisfactory for use in engine components, since they cannot display, simultaneously, high wear resistance and suitable toughness for working within an internal combustion engine.
The present coating, developed by the applicant, after many studies and researches, is new and inventive activity over the others, and offers significant advantages such as easy deposition, excellent mechanical properties and competitive application cost, high wear resistance and toughness compatible with the demands of the new engines and low friction aligned with the demands of emissions from internal combustion engines.
The present coating may be applied to any sliding elements used in an internal combustion engine as piston rings, bearing shells, bearings, bushings, etc., while they have a ferrous base.